Throughout this disclosure, expression “diluent” denotes condensable gas (or a mixture of condensable gases) or liquid present in a polymerization reactor with polymer resin being produced. The expression “diluent gas” (or “condensable diluent” or “condensable diluent gas”) denotes condensable gas (or a mixture of condensable gases) present in a polymerization reactor with polymer resin being produced. The diluent gas is condensable at the temperatures encountered in the process heat exchanger. Examples of diluents include induced condensing agents (“ICAs”), comonomers, isomers of comonomers, and combinations thereof.
The expression “dry polymer resin” (or “dry version” of polymer resin) is used herein to denote polymer resin that does not contain substantial amounts of dissolved gas. An example of dry polymer resin is polymer that had been previously produced in a polymerization reactor and then purged to eliminate all (or substantially all) unreacted monomers, comonomers and ICAs that had been dissolved in the polymer at the time of production. As will be discussed herein, a dry version of polymer resin has significantly different melting (and sticking) behavior than would the same polymer resin if it were in the presence of a significant amount of condensable diluent gas and comonomer.
The expression “optimal” diluent concentration (e.g., “optimal” ICA concentration) for a polymer resin-producing polymerization reaction herein denotes a diluent concentration that achieves or is expected to achieve a desired production rate for the reaction subject to at least one predetermined constraint (e.g., at least one constraint on properties of the polymer resin being produced and/or constraints on reactant and catalyst feed rates, cooling capacity, and reactor temperature). For example, in some cases an “optimal” diluent (e.g., ICA) concentration is a concentration of the diluent (e.g., ICA) which maximizes production rate with reactant and catalyst feed rates within predetermined limits with an acceptably low risk of occurrence of a reactor sheeting, or of a discontinuity event, or of an undesirable production rate constraint due to reduced resin flowability. For another example, an “optimal” diluent (e.g., ICA) concentration is a concentration of the diluent (e.g., ICA) which maximizes production rate without any constraint against occurrence of reactor sheeting or a discontinuity event (or a downstream flowability limitation).
The expression “on-line generation” of a value (or data) during a reaction herein denotes generation of the data sufficiently rapidly so that the value or data is available essentially instantaneously for use during the reaction. The expression “generation of” a value or data “in on-line fashion” during a reaction is used synonymously with the expression on-line generation of a value (or data) during a reaction. Generation of data from a laboratory test (on at least one substance employed or generated in the reaction) is not considered “on-line generation” of data during the reaction, if the laboratory test consumes so much time that parameters of the reaction may change significantly during the time required to conduct the test. It is contemplated that on-line generation of data may include the use of a previously generated database that may have been generated in any of a variety of ways including time-consuming laboratory tests.
With reference to a product being produced by a continuous reaction, the expression “instantaneous” value of a property of the product herein denotes the value of the property of the most recently produced quantity of the product. Because of the back-mixed nature of a gas phase polymerization reactor, the most recently produced polymer product typically undergoes mixing with previously produced quantities of product before a mixture of the recently and previously produced product exits the reactor. In contrast, with reference to a product being produced by a continuous reaction, the expression “average” (or “bed average”) value (at a time “T”) of a property herein denotes the value of the property of the product that exits the reactor at time T.
The expression “polyethylene” denotes at least one polymer of ethylene and optionally one or more C3-C10 α-olefins, while the expression polyolefin denotes at least one polymer (or copolymer) of one or more C2-C10 α-olefins.
Throughout this disclosure, the abbreviation “MI” denotes melt index (I2) of the polymer product, measured in accordance with ASTM-D-1238-E unless otherwise stated. Also throughout this disclosure, the term “density” denotes the intrinsic material density of a polymer product (in units of g/cc), measured in accordance with ASTM-D-1505-98 unless otherwise stated.
One method for producing polymers is gas phase polymerization. A conventional gas phase fluidized bed reactor, commonly employs a fluidized dense-phase bed typically including a mixture of reaction gas, polymer (resin) particles, catalyst, liquid additive(s), and (optionally) continuity aids (for improving fluidization) and/or other additives. Typically, any of several process control variables will cause the reaction product to have certain, preferably desired, characteristics.
Generally in a gas-phase fluidized bed process for producing polymers from monomers, a gaseous stream containing one or more monomers is continuously passed through a fluidized bed under reactive conditions in the presence of an activated catalyst. This gaseous stream is optionally withdrawn from the top of the fluidized bed and recycled back into the reactor. Simultaneously, polymer product is withdrawn from the reactor and new monomer is added to replace the polymerized monomer. The recycled gas stream is heated in the reactor by the heat of polymerization. This heat is typically removed in another part of the cycle by a cooling system external to the reactor.
It is important to remove heat generated by the reaction in order to maintain the temperature of the resin and gaseous stream inside the reactor at a temperature below the polymer melting point and/or catalyst deactivation temperature. Further, heat removal is important to control the reactor temperature to prevent excessive stickiness of polymer particles that if left unchecked, may result in loss of fluidization or agglomeration of the sticky particles which may lead to formation of chunks or sheets of fused polymer that cannot be removed as product. Excessive stickiness left unchecked may also limit reactor rates due to flowability limitations in the product discharge system, product conveying, product purging, rotary feeders, or other downstream process equipment.
The production of such chunks or sheets can present significant operational problems in fluidized bed reactor systems because, once formed, the fused chunks or sheets may fall onto the distributor plate causing impaired fluidization and mixing, which in many cases requires a reactor shutdown for cleaning. Prevention of such excessive resin stickiness has been accomplished by controlling the temperature of the fluid bed to a temperature just below the fusion or sintering temperature of the polymer particles. Above this fusion or sintering temperature, empirical evidence suggests that such fusion or sintering leads to agglomeration or stickiness in the polymer product, which can in turn (if left unchecked) lead to the above conditions.
In addition, the amount of polymer produced in a fluidized bed polymerization process is directly related to the amount of heat that can be withdrawn from the fluidized bed reaction zone. In steady state operation of the reaction process, ideally, the rate of heat removal from the fluidized bed must equal the rate of rate of heat generation, such that the bed temperature remains constant. Conventionally, heat has been removed from the fluidized bed by cooling the gas recycle stream in a heat exchanger external to the reactor.
A requirement of a fluidized bed process is that the velocity of the gaseous recycle stream be sufficiently high to maintain the reaction zone in a fluidized state. In a conventional fluidized bed polymerization process, the amount of fluid circulated to remove the heat of polymerization is greater than the amount of fluid required for support of the fluidized bed and for adequate mixing of the solids in the fluidized bed. The excess velocity provides additional gas flow to (and through) the fluid bed for additional cooling capacity and more intensive mixing of the reactor bed. However, to prevent excessive entrainment of solids in a gaseous stream withdrawn from the fluidized bed, the velocity of the gaseous stream must be regulated.
For a time, it was thought that the temperature of the gaseous stream external to the reactor, otherwise known as the recycle stream temperature, could not be decreased below the dew point of the recycle stream without causing problems of polymer agglomeration or plugging of the reactor system. The dew point of the recycle stream is that temperature at which liquid condensate first begins to form in the gaseous recycle stream. The dew point may be calculated knowing the gas composition, and may be thermodynamically defined.
Contrary to this belief, as suggested by Jenkins et al. in U.S. Pat. Nos. 4,543,399 and 4,588,790, a recycle stream can be cooled to a temperature below the dew point in a fluidized bed polymerization process resulting in condensing a portion of the recycle gas stream. The resulting stream containing entrained liquid can be returned to the reactor without causing the aforementioned agglomeration and/or plugging phenomena (which was generally expected prior to Jenkins). The process of purposefully condensing a portion of the recycle stream is known in the industry as “condensed mode” operation in a gas phase polymerization process.
The above-cited U.S. patents to Jenkins et al. suggest that when a recycle stream temperature is lowered to a point below its dew point in “condensed mode” operation, an increase in polymer production is possible, as compared to production in a non-condensing mode because of increased cooling capacity. Consequently, a substantial increase in space-time yield, the polymer production rate per unit of reactor volume, can be achieved by condensed mode operation with little or no change in product properties.
Cooling of the recycle stream to a temperature below the gas dew point temperature produces a two-phase gas/liquid mixture with solids contained in both of these phases. The liquid phase of this two-phase gas/liquid mixture in condensed mode operation remains entrained or suspended in the gas phase portion of the mixture. Vaporization of the liquid occurs only when heat is added or pressure is reduced. In this process, vaporization occurs when the two-phase mixture enters the fluidized bed, with the (warmer) resin providing the required heat of vaporization. The vaporization thus provides an additional means of extracting heat of reaction from the fluidized bed. The heat removal capacity is further enhanced in condensed mode operation by the increased (sensible) heat transfer associated with the lower temperatures of the gas stream entering the fluidized bed. Both of these factors increase the overall heat removal capability of the system and thereby enable higher space-time yields.
Jenkins, et al. illustrate the difficulty and complexity of such condensed mode reaction control in general, and of trying to extend the stable operating zone to optimize the space time yield in a gas phase reactor, especially when operating in condensed mode.
The cooling capacity of recycle gas can be increased further while at a given reaction temperature and a given temperature of the cooling heat transfer medium. One option for doing so is to add non-polymerizing, non-reactive materials to the reactor, which are in the gaseous state in the fluidized bed section of the reactor, but are condensable at the lower temperatures encountered in the process heat exchanger. Such non-reactive, condensable materials are collectively known as induced condensing agents (ICAs) since they “induce” additional condensing in the system. Increasing concentrations of ICA in the reactor cause corresponding increases in the dew point temperature of the reactor gas, which (for a given heat exchanger temperature) promotes higher levels of condensing for higher heat-transfer limited production rates for the reaction. Suitable ICA materials are selected based on their specific heat and boiling point properties. In particular, ICA compounds are selected such that a relatively high portion of the material is condensed at the cooling water temperatures available in polymer production plants, which are typically 20-40° C. ICA materials include hexane, isohexane, pentane, isopentane, butane, isobutane and other hydrocarbon compounds that are similarly non-reactive in the polymerization process.
U.S. Pat. No. 5,352,749, teaches among other things that there are limits to the concentrations of condensable gases, whether ICA materials, comonomers or combinations thereof, that can be tolerated in the reaction system. Above certain limiting concentrations, the condensable gases can cause a sudden loss of fluidization in the reactor, and a consequent loss in ability to control the temperature in the fluid bed. U.S. Pat. Nos. 5,352,749, 5,405,922 and 5,436,304, suggest upper limits of ICA in the reactor, depending on the type of polymer being produced. The authors characterized the upper limit of condensable materials by tracking the ratio of fluidized bulk density to settled bulk density. As the concentration of isopentane (ICA) was increased in an otherwise steady-state reaction, they found that the bulk density ratio steadily decreased. When the concentration of isopentane was sufficiently high, corresponding to a bulk density ratio of 0.59, they found that fluidization in the reactor was lost. They, therefore, determined that this ratio (0.59) represented a limiting value below which a reactor would cease functioning due to loss of fluidization.
As described in U.S. Pat. No. 7,122,607, attempts to operate polymerization reactors with excessive ICA concentrations cause polymer particles suspended in the fluid bed to become cohesive or “sticky,” and in some cases cause the fluid bed to solidify in the form of a large chunk. This stickiness problem is characterized by undesirable changes in fluidization and mixing in the fluid bed, which if left unchecked, may develop into a reactor discontinuity event, such as sheeting or chunking. Chunks are solid masses of polymer that can form within the interior of the fluidized bed. Sheets are solid masses of polymer that can form on the interior reactor walls. The sheets eventually become dislodged from the walls and fall into the reaction section. These solid masses of polymer (sheets or chunks) may settle on the distributor plate, where they interfere with fluidization, block the product discharge port, and usually force a reactor shut-down for cleaning and/or limit production rates until the reactor can be shut down. The lost production associated with such forced reactor shut-downs can have significant economic impact in large-scale, commercial production plants.
At least two distinct types of sheets can be formed in gas phase reactors: wall sheets or dome sheets, depending on where they are formed in the reactor. Wall sheets are formed on the walls (generally vertical walls) of the reaction section. Dome sheets are formed higher in the reactor, on the conical section of the dome, or on the hemispherical head on the top of the reactor.
When sheeting occurs with Ziegler-Natta catalysts, it is generally wall sheeting in the lower portion of the reaction section. Ziegler-Natta catalysts are capable of forming dome sheets, but the occurrence is rare. With metallocene catalysts, however, sheeting can occur in either or both locations (i.e. both wall sheeting and dome sheeting can occur). Dome sheeting has been particularly troublesome with metallocene catalyst systems.
The expression “discontinuity event” is used herein to describe a forced disruption in the continuous operation of a polymerization reactor caused by sheeting (e.g., wall or dome sheeting), chunking, or fouling of the gas recycle system. The terms “sheeting” and/or “chunking” while used synonymously herein, may describe different manifestations of problems discussed herein. In either manifestation (sheeting or chucking), the excessive polymer stickiness may lead directly to a reactor discontinuity event with the associated loss production, or to a reduced production rate due to reduced resin flowability (e.g., an unacceptably low rate of withdrawing resin from the reactor due to reduced flowability of sticky resin). Herein, the expression “stickiness” is used to denote a property of resin that is sometimes alternatively referred to as softness, or cohesiveness, or stickiness.
The expression “limiting stickiness” of polymer resin in a reactor (during a polymerization reaction in the reactor in the presence of reaction and diluent gases) herein denotes a condition of the resin which presents an unacceptably high risk of imminent occurrence of a discontinuity event in the reactor, or an unacceptably high risk of imminent occurrence of an undesirable production rate constraint due to reduced resin flowability (e.g., an unacceptably low rate of withdrawing resin from the reactor due to reduced resin flowability). In addition to causing a low rate of withdrawing resin from the reactor, resin stickiness can also limit rates by limiting product conveying to the product purge bin or reduce product flow rates out of the purging equipment or other downstream equipment.
Throughout this disclosure, each of the terms “fusion temperature,” “sintering temperature,” and “sticking temperature” denotes a temperature in a reactor (during a polymerization reaction in the presence of reaction and diluent gases) at which polymer resin in the reactor reaches a condition of limiting stickiness, thereby presenting an unacceptably high risk of imminent occurrence of a discontinuity event in the reactor (or of an undesirable production rate constraint due to reduced resin flowability).
Adding to the complexity of control of stickiness while using ICAs, different polymer products vary widely in their ability to tolerate ICA materials, some having a relatively high tolerance (expressed in partial pressure of the ICA in the reactor). For example, some polymers can tolerate as much as 50 psia of ICA, while other polymers can tolerate only 5 psia or less. With these latter polymers, the heat-transfer limited production rates under similar conditions are substantially lower. Polymers which possess a more uniform comonomer composition distribution are known to have a higher tolerance to the partial pressure of the ICA in the reactor. Metallocene catalyst produced polymers are a good example of polymers with such a more uniform comonomer composition. However, at some point even these metallocene produced polymers reach a limiting ICA concentration that induces stickiness. The limiting ICA concentration depends on several factors in addition to the polymer type (which determines polymer properties such as melt index and density which affect stickiness and the limiting ICA concentration), including reactor temperature, comonomer type and concentration, etc. Further, with the effect of temperature, ICA level and comonomer levels all affecting on the onset of stickiness, determining the point at which sticking begins to occur has heretofore been difficult.
Even within the constraints of conventional, safe operation, control of such reactors is complex adding further to the difficulty and uncertainty of experimentation if one wishes to find new and improved operating conditions that might result in higher production rates. Discontinuity events at large-scale, gas phase polymer production plants are expensive. Further, risks associated with experimentation in such plants are high due to the high cost of reactor downtime. Therefore, it is difficult to explore design and operating boundaries experimentally in view of the costs and risks involved.
It would be desirable to provide a method of determining a stable operating condition for gas fluidized bed polymerization, especially if operating in condensed mode, to facilitate optimum design of the plant and the determination of desirable process conditions for optimum or maximum production rates in a given plant design.
It would also be desirable to have a mechanism in commercial gas-phase reactors to detect the onset of stickiness (in on-line fashion) that is a better or an earlier indicator of the onset of stickiness than are conventional techniques (e.g., monitoring the fluidized bulk density as described in U.S. Pat. No. 5,352,749). Such a mechanism would allow the operators (or automated control system) to determine when conditions of limiting stickiness were being approached, and enable them to take corrective action before discontinuity events (such as sheeting and chunking) occurred, while keeping the reactors at or near conditions of optimal (e.g., maximum) ICA concentration, permitting higher production rates with substantially less risk.
PCT Application WO 2005/113615 and related U.S. Pat. No. 7,122,607 describe the determination of a critical temperature below which resin in a polymerization reactor cannot become sticky, and use of this predetermined critical temperature to control the reactor. These references define “dry sticking temperature” of a polymer (to be produced in a fluidized bed polymerization reactor) as the temperature at which agglomeration within the bed or fouling on any surface of the reactor vessel begins to occur with the reactor operating at normal pressure and gas velocity, but in the presence of substantially pure nitrogen rather than the normal gas components. They define a liquid “melting point depression” as the temperature by which the melting point of the polymer in the reactor is depressed by liquid immersion of the polymer in the condensable diluent(s) (e.g., ICA and comonomer) to be used in the process. Because the measurements are carried out in the presence of each condensable diluent in a liquid (rather than gas) state, the determined liquid melting point depression represents the maximum amount by which the melting point can be depressed in a reactor operating in the gas phase with the same condensable diluent(s). The references also disclose the steps of determining the dry sticking temperature of a polymer to be produced, determining the liquid melting point depression for the reaction, and then operating the gas phase reactor process with a bed temperature below the “critical temperature” (defined as the dry sticking temperature minus the liquid melting point depression). The references teach that performing the reaction with the bed temperature below this critical temperature can prevent stickiness that could otherwise be induced in the resin due to high concentrations of condensable diluent(s).
The “critical temperature” (to be referred to herein as “CT”) disclosed in WO 2005/113615 and U.S. Pat. No. 7,122,697 is a property of a specific polymer (e.g., a polyolefin) produced by a polymerization reaction in a gas phase fluidized-bed reactor. The CT is a temperature in the fluid bed below which the polymer cannot become sticky regardless of the concentration of condensable diluent(s) in the reactor. Thus, if the reactor were operated with a temperature equal to or less than the CT to produce the polymer in the fluid bed, it would be impossible for the polymer to become sticky even at the maximum depression of the polymer sticking temperature (whereas the actual amount of depression of the polymer sticking temperature would depend on the actual concentration of condensible diluent(s) in the reactor). CT varies with the characteristics of a polymer (e.g., density and MI) but not with temperature and other reaction conditions of the polymerization reaction which produces the polymer.
The CT disclosed in U.S. Pat. No. 7,122,697 is the polymer's dry sticking temperature minus the maximum melting point depression that could occur due to the presence of condensable diluent(s) in the reactor. For example, the difference between dry and fully immersed (liquid) Differential Scanning Calorimeter (“DSC”) peak melting temperatures for the polymer is taken to be the maximum melting point depression, where the polymer dry sticking temperature is taken to be the DSC peak melting temperature of the dry polymer. The CT is typically not the same temperature as the temperature dMIT=ΔMIT defined in the MIT application discussed below. The value of dMIT depends on the concentration of condensable diluent(s) in a polymerization reactor during production of a polymer, and thus can vary as a function of time during the reaction as diluent concentration changes. Depending on the current value of dMIT, the reaction may be subject to a high or low risk of occurrence of reactor sheeting or another discontinuity event. In contrast, the CT for a polymer is a limiting value that bounds the set of all the possible dMIT values that can exist during production of the polymer.
Above-referenced U.S. Patent Application No. 60/842,747 (the “MRT application”) and U.S. Patent Application No. 60/842,719 (the “MIT application”), both filed on Sep. 7, 2006, describe methods for detecting conditions indicative of imminent occurrence of sheeting during polymerization reactions in fluid bed polymerization reactors, and optionally also controlling the reactions to prevent the occurrence of sheeting.
The MRT application describes a method including of the steps of: monitoring a polymerization reaction which produces a polymer resin in a fluid bed reactor, wherein a dry melt reference temperature is characteristic of melting behavior of a dry version of the polymer resin; and in response to data indicative of at least one monitored parameter of the reaction, determining, in on-line fashion, a reduced melt reference temperature characteristic of the melting behavior of the polymer resin as it exists in the reactor. The reduced melt reference temperature (MRTR) is at least substantially equal to the difference between the dry melt reference temperature and a melt reference temperature depression value, “D,” where D is a temperature by which the dry melt reference temperature is depressed by the presence of diluent that is present with the resin in the reactor. The method optionally also includes the steps of determining a stickiness control parameter (e.g., a ΔMRT value) from the reduced melt reference temperature, and controlling the reaction in response to the stickiness control parameter. Herein the notation “dMRT” will be used to denote a ΔMRT value of the type described in the MRT application.
The MIT application describes a specific method of applying the MRT method, including of the steps of:
(a) during a polymerization reaction in a fluid bed reactor which produces a polymer resin, measuring parameters of the reaction including at least reactor temperature, at least one resin property of the polymer resin, and concentration of at least one condensable diluent gas in the reactor;
(b) determining from the at least one resin property, using a predetermined correlation, a dry melt initiation temperature of a dry version of the polymer resin; and
(c) during the reaction, using a melt initiation temperature depression model to determine, in on-line fashion from at least some of the parameters measured in step (a) and the dry melt initiation temperature value, a reduced melt initiation temperature for the polymer resin in the presence of the at least one condensable diluent gas, said melt initiation temperature depression model identifying an estimated degree of depression of the dry melt initiation temperature due to presence of at least one diluent with the polymer resin.
In typical embodiments, the melt initiation temperature depression model implements the well-known Flory melt depression equation. The method optionally also includes the step of:
(d) determining in on-line fashion a temperature value indicative of resin stickiness in the reactor, from the reduced melt initiation temperature determined in step (c) and a current value of the reactor temperature.
Typically, the temperature value generated in step (d) is a temperature value ΔMIT that is at least substantially equal to Trx−MITR, where Trx is the current value of reactor temperature, and MITR is the reduced melt initiation temperature determined in step (c). Herein the notation “dMIT” will be used to denote a ΔMIT value of the type described in the MIT application.
The MIT and MRT applications describe control of a polymerization reaction to prevent a stickiness control parameter (e.g., a dMIT value or dMRT value) from exceeding a limiting value (e.g., a limiting dMRT or dMIT value) or leaving a limiting range, including by adjusting the reactor temperature or ICA concentration to bring the stickiness control parameter back into an acceptable range. Adjustments in the reactor temperature (rather than ICA concentration) would generally be preferred for this purpose because of the relatively quick response times involved. The MIT application notes, for example, that if the stickiness control parameter were too high by 1° C., a reduction in reaction temperature of 1° C. could bring the stickiness control parameter back within range within a few minutes. In some cases, an excessively high stickiness control parameter can be corrected by lowering the concentration of ICA in the reactor, e.g., by reducing the rate of ICA feed to or increasing the rate of ICA venting from the reactor. However, the desired change in ICA concentration would occur relatively slowly (several hours would typically be required to change ICA concentration to a desired level).
It has been proposed to optimize ICA concentration in a polymerization reactor to achieve a desired production rate subject to constraints. For example, U.S. Patent Application Publication No. 2007/0043174 A1, published on Feb. 22, 2007, discloses a method for controlling a gas phase reaction (e.g., a gas phase, fluidized bed polymerization reaction) which includes steps of calculating an optimal concentration of ICA (induced condensation agent) or induced cooling agent in the reactor and controlling the flow of ICA or cooling agent to the reactor in order to achieve the optimal ICA (or cooling agent) concentration. In some embodiments, the optimal ICA concentration is said to be one which maximizes the polymer production rate subject to predetermined constraints which may include (for example) constraints on reactor feed monomer flow rate and cooling water flow rate. When an optimal ICA concentration has been determined, the actual ICA concentration may be reduced to the optimal concentration when this would not reduce the production rate (which is otherwise constrained) or may be increased to the optimal concentration when this would increase the production rate, provided that neither such concentration change would violate another constraint on the reaction. For example, consistent with the reference's teaching that bed temperature in a fluid bed polymerization reactor should be controlled to remain below the sintering temperature of the polymer being produced, the ICA concentration may be constrained against being changed to a level that causes the bed temperature to increase above the sintering temperature.
To calculate optimal ICA concentration, ICA limits must be taken into account including maximum and minimum ICA concentration based on reactor inlet dew point and condensing limits, as well as maximum ICA concentration to avoid resin stickiness. Excessive ICA concentrations have been shown to increase resin stickiness unacceptably which can reduce resin flowability and therefore reactor throughput, and can cause resin agglomeration or a discontinuity event. Violation of the maximum or minimum ICA concentration limit can lead to unwanted process upsets including loss of production capacity, offspec material, or possibly even reactor shutdown.
Conventional ICA optimization routines attempt to control ICA concentration to a level that provides sufficient cooling capacity for the desired production rate and cooling requirement while minimizing excess ICA, in an effort to maximize reactor throughput while minimizing ICA usage and improving raw material efficiency. It is known to maximize polymerization production rate by selecting an optimal ICA concentration subject to constraints, including the constraint of avoiding resin stickiness. Conventional ICA optimization programs provide for a ICA stickiness limit (e.g., a limiting value of ICA concentration beyond which resin stickiness may occur) to be input into the calculation routines. However, the resin stickiness models and correlations employed to perform conventional ICA optimization have been inadequate (so that the true ICA stickiness limit is not known) and have lacked the accuracy needed for controlling ICA concentration at or near the true stickiness limit. Due to the high risk and severe consequence of violating the maximum ICA stickiness limit and the poor accuracy of the stickiness models employed, conventional ICA optimization routines have used very conservative estimates of the maximum ICA concentration (for avoiding stickiness). These conservative constraints on allowable ICA concentration have typically been based on very limited operating data and their use has led to failure to achieve maximum reactor throughput (due to overlimiting of ICA concentration) as well as loss of raw materials when too much ICA is vented during transitions in order to operate within the excessively conservative ICA concentration limits. Although the true ICA stickiness limit is dependent on many dynamic process variables, conventional ICA optimization routines have also employed a constant ICA stickiness limit per product recipe.
It would be desirable to monitor a gas phase, fluidized bed polymerization reaction to obtain data that provide:
an early indication of impending sheeting or other discontinuity event (or of imminent occurrence of an undesirable production rate constraint due to reduced resin flowability) in the reactor that would provide sufficient advanced warning to enable the operators to make changes in the process to avoid the impending discontinuity event or production rate constraint; and
an accurate estimate of limit(s) on allowable ICA concentration for use in determining optimal ICA concentration for the reaction.
Preferably, such monitoring would be done in an on-line fashion (e.g., during fluid bed polymerization reactions that employ metallocene catalysts, based on real-time measurements of the process parameters of that cause wall and dome sheeting when such catalysts are employed).
It would also be desirable to control a fluid bed polymerization reaction to maximize production rate by controlling temperature and/or diluent (e.g., ICA) concentration in response to an accurately determined optimal diluent (e.g., ICA) concentration (e.g., by reducing reactor temperature below the level that would have conventionally been considered a threshold and/or increasing ICA concentration above the level that would have conventionally been considered a threshold when appropriate).